ID shortener in Golang

shortener.go

package shortener

import (
	"strings"
)

var shortenerDict = `abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789` // you can customize this, just make sure there's no duplicate chars
var shortenerBase = len(shortenerDict)

func ShortenID(ID int64) string {
	result := ""

	for ID > 0 {
		mod := ID % int64(shortenerBase)
		result = shortenerDict[int(mod):int(mod)+1] + result
		ID /= int64(shortenerBase)
	}

	return result
}

func ExpandShortID(shortID string) int64 {
	var id int

	for i, char := range shortID {
		index := strings.Index(shortenerDict, string(char))
		id += index

		if i < len(shortID)-1 {
			id *= shortenerBase
		}
	}

	return int64(id)
}